Method for adaptively allocating resources in communication system

ABSTRACT

A method for adaptively allocating resource can be simply implemented for reducing degradation of performance by effectively separating operations of sub-channel and time slot allocation and selection of modulation method and sequentially processing each of operations. A method for adaptively allocating resource in a communication system to sequentially process sub-carrier/time slot allocation and modulation method selection efficiently includes the step of a) computing average channel gains of sub-carriers/time slots for each user by using channel gains of sub-carriers/time slots for each user; b) computing average numbers of bits for each user by using required data rates and average channel gains of sub-carriers/time slots for each user; c) computing the number of sub-carriers/time slots allocated to each user and allocating the sub-carriers/time slots to each user; and d) selecting a modulation method with respect to each sub-carrier/time slot.

FIELD OF THE INVENTION

[0001] The present invention relates to a method for adaptivelyallocating resources in a communication system; and, more particularlyto a method for adaptively allocating resources in an orthogonalfrequency division multiple access (OFDMA)/time division multiple access(TDMA) system and a computer readable medium on which a program forexecuting the same method is recorded.

DESCRIPTION OF THE PRIOR ART

[0002] Recently, transmission of data and video through the Internet hasbeen popular and transmission amount of data and video also incrediblyincreased. Accordingly, a high-speed data transmission also has beendemanded. For transmission of data and video in high speed, a frequencyhas to be used effectively.

[0003] However, the frequency is shared by each of users in anorthogonal frequency division multiple access (OFDMA) system. Thus, ithas to be considered to use a time division multiple access (TDMA) forallocating resources in the OFDMA system in order to increaseflexibility of resource allocation and reduce waste caused by a pilotsignal when the number of users increases. It is referred to as a hybridOFDMA/TDMA system in this specification.

[0004] A typical OFDM system uses the same modulation method for everysub-carrier regardless of a magnitude of a channel response provide fromeach sub-carrier. The hybrid OFDMA/TDMA system also allocates users withan arbitrary sub-carrier and an arbitrary time slot regardless of themagnitude of each user's channel response.

[0005] When the channel response of each sub-carrier in the transmitteris estimated, the OFDM system properly divides power so that a powergain can be obtained by using a different modulation method to eachsub-carrier according to the channel response.

[0006] Also, if proper time slots and sub-carriers are allocated in theOFDM system by considering the channel response of each user and theadaptive selection of modulation method, an additional power gain can beobtained.

[0007] However, the above described methods are too complicated to beimplemented because the allocation of time slot and sub-channel of eachuser and the selection of modulation method for the allocation aresimultaneously operated in the hybrid OFDMA/TDMA.

SUMMARY OF THE INVENTION

[0008] It is, therefore, an object of the present invention to provide amethod for adaptively allocating resources which can be simplyimplemented for reducing degradation of performance by effectivelyseparating operations of sub-channel and time slot allocation andselection of modulation method and sequentially processing each ofoperations.

[0009] Also, it is another object of the present invention to provide acomputer readable recording medium on which a program for executing thesame method is recorded.

[0010] In accordance with an aspect of the present invention, there isprovided a computer readable recording medium for storing instructionsfor executing a method for adaptively allocating resource in acommunication system including a microprocessor by subsequentlyprocessing sub-carrier/time slot allocation and modulation methodselection, comprising the methods of: a) computing average channel gainsof sub-carriers/time slots for each user by using channel gains ofsub-carriers/time slots for each user; b) computing average numbers ofbits for each user by using required data rates and average channelgains of sub-carriers/time slots for each user; c) computing the numberof sub-carriers/time slots allocated to each user and allocating thesub-carriers/time slots to each user; and d) selecting a modulationmethod with respect to each sub-carrier/time slot.

[0011] In accordance with another aspect of the present invention, thereis provided a computer readable recording medium including amicroprocessor for allocating an adaptive source in a communicationsystem, including the instructions of: a) computing average channelgains of sub-carriers/time slots for each user by using channel gains ofsub-carriers/time slots for each user;; b) computing average numbers ofbits for each user by-using required data rates and average channelgains of sub-carriers/time slots for each user; c) computing the numberof sub-carriers/time slots allocated to each user and allocating thesub-carriers/time slots to each user; and d) selecting a modulationmethod with respect to each sub-carrier/time slot.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The above and other objects and features of the present inventionwill become apparent from the following description of the preferredembodiments given in conjunction with the accompanying drawings, inwhich:

[0013]FIG. 1A is a block diagram showing an adaptive resource allocatorin a transmitter of a base station in accordance with the presentinvention;

[0014]FIG. 1B is a block diagram illustrating an adaptive resourceallocator in a receiver of a mobile terminal in accordance with thepresent invention;

[0015]FIG. 2 is a conceptual view of resources to be used by users in ahybrid OFDMA/TDMA system;

[0016]FIG. 3 is a flowchart for explaining a method for adaptivelyallocating resource in a communication system in accordance with apreferred embodiment of the present invention;

[0017]FIGS. 4A to 4D are graphs showing channel responses of user inaccordance with another preferred embodiment of the present invention;

[0018]FIG. 5 is an exemplary drawing showing results of sub-carrierallocations in accordance with the present invention; and

[0019]FIG. 6 is an exemplary drawing showing results of users selecteddifferent methods of modulation in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0020] Other objects and aspects of the invention will become apparentfrom the following description of the embodiments with reference to theaccompanying drawings, which is set forth hereinafter.

[0021]FIG. 1A is a block diagram showing an adaptive resource allocatorin a transmitter of a base station in accordance with the presentinvention. FIG. 1B is a block diagram illustrating adaptive resourceallocator in a receiver of a mobile terminal, especially, a k-th mobileterminal in accordance with the present invention.

[0022] Referring to FIGS. 1A and 1B, an inverse fast fourier transformer(IFFT) 130 and a fast fourier transformer (FFT) 160 of a typicalorthogonal frequency division multiplexing (OFDM) or orthogonalfrequency division multiple access (OFDMA) system, is used in thetransmitter and the receiver of the present invention.

[0023] Signals transmitted from the IFFT 130 of the transmitter to theFFT 160 of the receiver are sub-carriers and each sub-carrier channelcan be easily modeled by multiplying a channel gain withoutinterference.

[0024] That is, a K-th signal of the FFT 160 is obtained by multiplyinga K-th signal of the IFFT 130 and a channel gain of the K-th signal,which is different for each sub-carrier.

[0025] Therefore, if the channel gain of the each sub-carrier isobtained, the transmitter can send more data on a sub-carrier having alarge channel gain and send less data on a sub-carrier having a smallchannel gain. This scheme is an adaptive modulation.

[0026] In the OFDMA system, different users can transmit data ondifferent sub-carriers and time slots and in the OFDMA system using afixed resource allocation, users can transmit data on arbitrarilysub-carriers and time slots.

[0027] However, the channel gains are different among each sub-carrierand also different among each user. That is, in order to have more powergain, a better channel can be allocated to a user considering thechannel gain because the user may have a small channel gain on onechannel and may have a large channel gain on other channel.

[0028] It is explained that a hybrid orthogonal frequency divisionmultiple access (OFDMA)/time division multiple access (TDMA) system hasthe same problem of the OFDMA system and a method for adaptiveallocation of sub-carriers and adaptive modulation is suggested in thespecification of the present invention.

[0029] Referring to FIG. 1A, the hybrid OFDMA system includes asub-carrier allocation and modulation method selection unit 110. Anadaptive modulator 120 is coupled to the sub-carrier allocation andmodulation method selection unit 110 and modulates user data adaptivelyaccording to the data received from the sub-carrier allocation andmodulation method selection unit 110.

[0030] Referring to FIG. 1B, sub-carrier allocation and modulationinformation are transferred to a sub-carrier selector and adaptivedemodulator 170 through a control channel for sub-carrier selection andadaptive demodulation.

[0031]FIG. 2 is a conceptual view of resources to be used by users inthe hybrid OFDMA/TDMA system.

[0032] Referring to FIG. 2, it will be described in detail that theproblem of adaptive resource allocation of the hybrid OFDMA/TDMA systemis similar to that of the OFDMA system.

[0033] As shown in FIG. 2, in case that the number of time slots is Tand the number of sub-carriers is N, the number of available resourcesbecomes NT.

[0034] When K number of users are transmitting data through NT number ofresources, it is determined how the NT number of resources are allocatedto K number of users, what kind of modulation method is used and howmuch power is used to transmit the data in accordance with the presentinvent. It is assumed that the hybrid OFDMA/TDMA has NT number ofsub-carriers because data are independently transmitted on sub-carriersand time slots. It is also assumed that T sets of N number of channelgains are the same because the channel gains of the hybrid OFDMA/TDMA inaccordance with the present invention are time-invariant. Therefore, thehybrid OFDMA/TDMA in accordance with the present invention can beassumed as the OFDMA system that has NT number of sub-carriers.

[0035]FIG. 3 is a flowchart for explaining a method for adaptivelyallocating resource in a communication system in accordance with apreferred embodiment of the present invention.

[0036] At Step S301, when it is assumed there are K numbers of users andN numbers of sub-carriers, channel gains α² _(k,n)(k=1, . . . , K, n=1,. . . , N) of sub-carriers for each user and data rates R_(k)(k=1, . . ., K) for each user are obtained.

[0037] An N is a multiple of the number of channels and the number oftime slots in the hybrid OFDMA/TDMA system.

[0038] A channel gain is obtained by estimating the channel and arequired data rate is obtained by exchanging information between a basestation and a mobile terminal.

[0039] At Step S303, average channel gains of sub-carriers for each userare computed by using the channel gains of sub-carriers for each user inaccordance with following equation 1. $\begin{matrix}{{{\overset{\_}{\alpha}}_{k}^{2} = {\frac{1}{N}{\sum\limits_{n = 1}^{N}\quad \alpha_{k,n}^{2}}}},\quad {{{for}\quad k} = {1\quad \ldots \quad K}}} & \lbrack {{Equation}\quad 1} \rbrack\end{matrix}$

[0040] At Step S305, average numbers of bits {overscore (c)}_(k) foreach user are computed by using the average channel gains and therequired data rates for each user.

[0041] {overscore (c)}_(k) is obtained from below equation 2 formulatedby adding a proper assumption into an optimal solution disclosed in anarticle by C. Y. Wong, et al, “Multiuser OFDM with Adaptive Sub-carrier,Bit, and Power Allocation,” IEEE Journal of Selected Areas onCommuncations, pp. 1743˜1758, 1999. $\begin{matrix}\begin{matrix}{{\frac{{{\overset{\_}{c}}_{k}{f^{'}( {\overset{\_}{c}}_{k} )}} - {f( {\overset{\_}{c}}_{k} )}}{{\overset{\_}{\alpha}}_{k}^{2}} = ɛ},\quad {{{for}\quad k} = 1},{\ldots \quad K}} \\{{\sum\limits_{k = 1}^{K}\quad {R_{k}\text{/}{\overset{\_}{c}}_{k}}} = N}\end{matrix} & \lbrack {{Equation}\quad 2} \rbrack\end{matrix}$

[0042] Although the number of bits for each sub-carrier is computed byusing the channel gain of each sub-carrier in the prior optimalsolution, the number of bits for each sub-carrier is computed by usingthe average channel gain of sub-carrier for each user in the equation 2.Therefore, the sub-carriers allocation is taken apart from bitsallocation.

[0043] In the equation 2, ƒ(c) is the power to receive data within a biterror rate of c bits.

[0044] The power ƒ(c) varies in accordance with methods of modulation,e.g., quadrature amplitude modulation (QAM), phase shift keying (PSK),pulse amplitude modulation (PAM). In case of QAM, the power ƒ(c) isdescribed as equation 3.

ƒ(c)=N ₀ [Q ⁻¹(p _(e)/4)]²(2^(c)−1)/3   [Equation 3]

[0045] In the equation 3, N₀/2 is a variance of white gauss noise, p_(e)is a bit error rate, and Q(x) is a Q function.

[0046] The equation 3 is a convex function, which can be applied to QAM,PSK, and PAM.

[0047] If a non-linear equation with K+1 equations and variables of theequation 2 is a convex function, there always exist an optimal solution.Therefore, {overscore (c)}_(k) can be obtained by a Newton methoddisclosed by K. E. Atkinson, Numerical Analysis, Wiley & Sons Inc.,1998.

[0048] At Step S307, a total number of sub-carriers for each user iscomputed by equation 4. A total number of bits is R_(k) in belowequation 4.

n_(k)=R_(k)/{overscore (c)}_(k), for k=1, . . . , K   [Equation 4]

[0049] If the average number of bits {overscore (c)}_(k) from theequation 2 and the number of sub-carriers n_(k) of each user from theequation 4 are substituted into the optimal solution, a simplifiedoptimal solution is obtained as equation 5. $\begin{matrix}\begin{matrix}{{{Minimize}\quad P_{i}} - {\sum\limits_{k = 1}^{K}\quad {\sum\limits_{n = 1}^{N}\quad {r_{k,n}\rho_{k,n}}}}} \\{{{{Subject}\quad {to}\quad {\sum\limits_{n = 1}^{N}\quad \rho_{k,n}}} = n_{k}},{{for}\quad {all}\quad k}} \\{{{\sum\limits_{k = 1}^{K}\quad \rho_{k,n}} = 1},{{for}\quad {all}\quad n}}\end{matrix} & \lbrack {{Equation}\quad 5} \rbrack\end{matrix}$

[0050] In the equation 5, ρ_(k,n) is a binary variable, which shows ifthe K-th user uses the n-th sub-carrier. The value is 1 for using and 0for not using.

[0051] That is, at step S311, the sub-carriers are allocated by solvingthe optimal solution of the equation 5.

[0052] An r_(k,n) of the equation 5 is the cost for the K-th user to usethe n-th sub-carrier, and this relationship is further described in thebelow equation 6.

r _(k,n)=ƒ({overscore (c)} _(k))/α² _(k,n), for k=1 , . . . , K and n=1,. . . , N   [Equation 6]

[0053] Equation 5 is basically an integer type optimal solution becauseof the binary variable ρ_(k,n). Although the computation is complicatedto obtain the integer type optimal solution, the equation 5 is aparticular solution, which can be solved with the integer condition ofρ_(k,n) removed.

[0054] Although the optimal solution can be solved with a Simplexmethod, it can not be practically implemented in real time.

[0055] When the optimal solution of the equation 5 is reviewed forminimizing the complexity, it is regarded as a transportation problemwhich is a particular solution of a linear optimal problem. In thiscase, N sub-carriers are suppliers and K users are consumers. The firstconstraint is that each consumer demands n_(k) items. The secondconstraint is that all suppliers supply only one item.

[0056] This type of transportation problem becomes a very simplecomputation with Vogel's method. The solution has small performancedegradation compared to the optimal solution. The Simplex Method and theVogel's Method are disclosed by W. L. Winston, entitled OperationsResearch, Duxbury Press, 1994.

[0057] At Step S311, the sub-carriers are allocated to each user by theequation 5 and at Step S313, a modulation method for each user isselected.

[0058] Once the sub-carrier allocation is finished, a condition that aplurality of users transmits data on one sub-carrier is removed. Then,bits are allocated on the allocated channel of each user by using themodulation method for an individual OFDM user.

[0059] The sub-carrier allocation and the modulation method selectionare described in the preferred embodiment of the present invention.Those skilled in the art will also obviously find out that theallocation and the modulation method selection for time slots areperformed in the same manner as those for the sub-carrier.

[0060] FIGS. 4 to 6 are graphs for describing the preferred embodimentof the present invention, wherein there are 4 users, 64 sub-carriers,and 8 time slots.

[0061]FIGS. 4A to 4D are graphs showing channel responses of user inaccordance with another preferred embodiment of the present invention.

[0062] Each user has a different average channel gain in accordance witha distance estimated from the base station. The average channel gains ofeach user are shown as 0.0025, 0.3922, 1.3452 and 2.2601 in thepreferred embodiment of the present invention.

[0063] As shown in FIGS. 4A to 4D, the sub-carriers have differentvalues according to each user, but the values do not vary with the timeslots. Therefore, the channel response of the first channel for user 1is the same value with respect to the time slots 1 to 8. Total numbersof allocated channels for each of the 4 users computed from the equation1 to 4 are 297, 83, 67, 62, respectively. The total number of channelsis multiple of the number of sub-carriers and the number of time slots,which is 512.

[0064] The user 1 has a relatively large number 297 of the sub-carriersbecause the user 1 has a low average channel response 0.0025. The user 4has the least number 62 of the sub-carriers because the user 4 has thehighest average channel response 2.2601.

[0065] The optimal solution of the equation 5 can be solved with thesenumbers of the allocated sub-carriers of each user.

[0066]FIG. 5 is an exemplary drawing showing results of sub-carrierallocations in accordance with the present invention.

[0067] The dark mark shows the allocated channel to the user and thewhite mark shows the channel which is not allocated to the user.

[0068]FIG. 6 is an exemplary drawing showing results of users selecteddifferent methods of modulation in accordance with the presentinvention.

[0069] The white mark represents the channel which bits are notallocated while the dark mark represents the channel which bits areallocated. Particularly, the darker marks, the more bits are allocatedin the channel. QPSK is 2 bits transmission and 16QAM is 4 bitstransmission.

[0070] Comparing FIGS. 5 and 6, the bits are allocated on each user'sallocated sub-carrier in accordance with a magnitude of thesub-carriers. The users having a large average channel gain areallocated with the small number of sub-carriers, which transmit thelarge number of bits. The user 1 is allocated with the large number ofsub-carriers and using low order modulation methods because of a poorchannel response.

[0071] All users have channels which may not have any bit during themodulation method selection according to the channel magnitude as shownin FIG. 5.

[0072] Table 1 shows a performance difference between the optimalsolution proposed by the prior art disclosed in the previously mentionedarticle and the present invention with 4 users and 64 sub-carriers.TABLE 1 Power of the Data Rate Power of the suboptimal solution(bits/OFDM optimal solution of the present symbol) of the prior artinvention 128 27.62 dB 27.54 dB 256 34.49 dB 34.41 dB 384 40.56 dB 40.50dB

[0073] Table 1 is an averaged result of 1000 times performed trials toget an average performance difference.

[0074] It is assumed that required data rates of each user areidentical. The total data rate is varied from 128 bits/OFDM symbol to384 bits/OFDM symbol. In case of 128 bits/OFDM symbol, QPSK is usedwithout the adaptive modulation because the number of sub-carriers is64.

[0075] The present invention offers significant simplicity compared tothe optimal solution of the prior art while incurring small performancedegradation of 0.6-0.8 dB according to Table 1.

[0076] As a result, the present invention can execute the adaptivesub-carrier/time-slot allocation and the modulation method selectionwith the computation, which is practical to be implemented in the hybridOFDMA/TDMA system.

[0077] Also, the present invention obtains significant power gaincompared to the prior fixed modulation method and increase theefficiency of frequency usage.

[0078] The method of the present invention can be implemented as aprogram and stored in computer readable medium, e.g., a CD-ROM, a RAM, aROM, a floppy disk, a hard disk and an optical/magnetic disk.

[0079] The present invention can efficiently execute allocation ofsub-carriers and time slots when the hybrid OFDMA/TDMA is used as multiaccessing method in a data communication system that is operated in theOFDM.

[0080] Also, the present invention obtains more power gain and is moreefficient in using frequencies than the conventional system that usesthe fixed modulation method.

[0081] While the present invention has been described with respect tocertain preferred embodiments, it will be apparent to those skilled inthe art that various changes and modifications may be made withoutdeparting from the scope of the invention as defined in the followingclaims.

What is claimed is:
 1. A method for adaptively allocating resource in acommunication system by subsequently processing sub-carrier/time slotallocation and modulation method selection, the method comprising thesteps of: a) computing average channel gains of sub-carriers/time slotsfor each user by using channel gains of sub-carriers/time slots for eachuser; b) computing average numbers of bits for each user by usingrequired data rates and average channel gains of sub-carriers/time slotsfor each user; c) computing the number of sub-carriers/time slotsallocated to each user and allocating the sub-carriers/time slots toeach user; and d) selecting a modulation method with respect to eachsub-carrier/time slot.
 2. The method as recited in claim 1, wherein theaverage channel gain of each user in the step a) is computed by using anequation as:${{\overset{\_}{\alpha}}_{k}^{2} = {\frac{1}{N}{\sum\limits_{n = 1}^{N}\quad \alpha_{k,n}^{2}}}},\quad {{{for}\quad k} = {1\quad \ldots \quad K}}$

wherein {overscore (α_(k))}² is the average channel gain of sub-carrierfor each user and α² _(k,n) is the channel gain of sub-carrier/time slotfor each user.
 3. The method as recited in claim 1, wherein the averagenumber of bits for each user in the step b) is a solution of K+1non-linear equations formulated by an equation as: $\begin{matrix}{{\frac{{{\overset{\_}{c}}_{k}{f^{'}( {\overset{\_}{c}}_{k} )}} - {f( {\overset{\_}{c}}_{k} )}}{{\overset{\_}{\alpha}}_{k}^{2}} = ɛ},{{{for}\quad k} = 1},\ldots \quad,K} \\{{\sum\limits_{k = 1}^{K}\quad {R_{k}\text{/}{\overset{\_}{c}}_{k}}} = N}\end{matrix}$

wherein {overscore (c)}_(k) is an average number of bits for each user,ƒ(c) is a power to receive c bits data within a range of bit error rate,and R_(k) is the total number of bits for each user.
 4. The method asrecited in claim 1, wherein the number of sub-carriers/time slots in thestep c) is computed by using an equation as: n_(k)=R_(k)/{overscore(c_(k))}, for k=1, . . . , K wherein, n_(k) is the number of allocatedsub-carriers/time slots for each user.
 5. The method as recited in claim1, wherein the allocation of sub-carrier/time slot in the step c) is anoptimal solution of an equation as: $\begin{matrix}{{{Minimize}\quad P_{i}} - {\sum\limits_{k = 1}^{K}\quad {\sum\limits_{n = 1}^{N}\quad {r_{k,n}\rho_{k,n}}}}} \\{{{{{Subject}\quad {to}\quad {\sum\limits_{n = 1}^{N}\quad \rho_{k,n}}} = n_{k}},{{for}\quad {all}\quad k}}\quad} \\{{{\sum\limits_{k = 1}^{K}\quad \rho_{k,n}} = 1},\quad {{for}{\quad \quad}{all}\quad n}}\end{matrix}$

wherein ρ_(k,n) is a variable number which determines whether a K-thuser uses an n-th sub-carrier and r_(k,n) is a cost for the K-th user touse the n-th sub-carrier.
 6. The method as recited in claim 5, whereinthe cost for the K-th user to use the n-th sub-carrier is determined byan equation as: r _(k,n)=ƒ({overscore (c)} _(k))/α² _(k,n), for k=1, . .. , K and n=1, . . . , N.
 7. The method as recited in claim 5, wherein alinear optimal solution is solved by applying a Vogel's method.
 8. Acomputer readable recording medium for storing instructions forexecuting a method for adaptively allocating resource in a communicationsystem including a microprocessor by subsequently processingsub-carrier/time slot allocation and modulation method selection,comprising the methods of: a) computing average channel gains ofsub-carriers/time slots for each user by using channel gains ofsub-carriers/time slots for each user; b) computing average numbers ofbits for each user by, using required data rates and average channelgains of sub-carriers/time slots for each user; c) computing the numberof sub-carriers/time slots allocated to each user and allocating thesub-carriers/time slots to each user; and d) selecting a modulationmethod with respect to each sub-carrier/time slot.